1. Field of the Invention
The present invention relates to a multi-mode renewable power converter system, in particular to a power converter that is able to operate with different modes to suit various power and load requirements. In addition, it can also improve conversion efficiency in some existing solar power conversion systems.
2. Description of Related Arts
The electrical power demand from industrial and household users continue to increase, amidst growing concerns of the pollution problems and the depletion of coal and oil in our planet. The technology for utilizing renewable power from the natural resources has received increasing attention by scientists around the world. Among all the alternatives, solar power appears to be most promising, because it is most abundant in our environment and the cleanest among all. People living in remote areas and satellite towns have been using solar cells to supplement their regular power supply. Many countries have encouraged their people to install the solar power system in their homes to reduce the burden on existing power utilities. Environmental issues such as nuclear waste and carbon dioxide emission from the thermal power plant are still unresolved up to the present. With so many benefits for using renewable energy, the power output from the renewable energy is directly connected to the electricity distribution grid in many countries.
However, conversion of solar energy in general is still very expensive, and the efficiency of inverters for converting DC output from solar cells to sinusoidal AC power is another challenge for producers of power generators. FIG. 7 shows the architecture of a conventional solar power generator, solar power collected by a solar panel is converted to AC power by an inverter. The system suited for a standalone load includes a charger (70), a boost converter (71), an inverter (72), and a storage battery (73). The charger (70) is connected to the output of the solar panel (80) for charging the battery (73) with the solar energy; the boost converter (71) is connected to the output of the charger (70) and the storage battery (73) for boosting battery voltage to higher voltage; the inverter (72) is connected to the boost converter (71) for converting the high DC power to AC output and then delivering the power to the load (81); the storage battery (73) is connected to the output of the charger (70).
In actual operation, the above mentioned solar power system uses a solar panel (80) to collect solar power and generates DC output to the charger (70) which then charges the storage battery (73), which is typically a 48V storage battery for supplying the power to a standalone load. Generally, for certain heavy duty applications, the voltage of the storage battery is not high enough, it needs additional conversion stages to boost the output voltage, but in that case, the total power conversion efficiency from renewable power source to AC output will not be very high (approx. 90%).
In some applications, to make the optimal usage of the power from photovoltaic (PV) array, a maximum power point tracking (MPPT) circuit is employed for tracking the optimum power output of the PV array. As shown in FIG. 6, this MPPT circuit function is mainly performed in the charger section (70) only. Although this can make the maximum use of PV array, the conversion efficiency cannot be improved.
In FIG. 8, the block diagram shows the architecture of another solar power system for high power grid-connected systems (81a). The basic structure of the system is similar to the previously described case, only without the storage battery and the charger (not shown). The power from the solar panel (80) is boosted to higher level directly and converted into AC output to the grid or utility (81a), wherein the solar panel (80) can be implemented by high voltage photovoltaic arrays (PV array) (>300V). In addition, the voltage level from PV array is generally higher, therefore the total conversion efficiency of the system can be raised to 95%. The shortcoming of such system is that it lacks a back-up power system. Even if a battery can be connected to the system output, the voltage is too high for ordinary user applications. An additional charger must be used resulting in increased circuit complexity and inconvenience to ordinary power users. It is apparent that in a more ideal situation the system needs to have an inverter that can match the different power specifications for standalone load or grid-connected system.